The present invention falls into the general category of electronic packaging and more specifically into processes meeting high density line requirements on ceramic substrates utilized in electronic packages.
Ceramic substrates such as alumina possess an inherent surface topographical variation, the peak to valley distance falling in the range of 4-7 .mu.m. Photolithographic processes applied to such surfaces do not give the highest resolution that the process is capable of. In addition, such surfaces can result in defects in the photoresist applied to the substrate surface. Chemical development of the photoresist often produces pinholes or regions of unetched photoresist. Consequently metallization of the substrate results in metallized islands at the location of the pinholes and "opens" at the location of unetched photoresist. As finer (i.e. narrower) metallized lines are made, the condition of the lines become worse. In addition to ceramic substrates, the surface problem also exists with printed circuit boards and porcelainized steel.
In the prior art there has been the use of the process of glazing of an alumina surface to provide a smoother surface, however this is a hot process utilizing temperatures of 600.degree. C.-950.degree. C.
The improvement comprises laminating over the substrate surface a dry film photosensitive material that withstands temperatures of molten solder and has a thickness of about 10 .mu.m or greater such as RISTON solder mask. The lamination is a dry process which laminates the solder mask in sheet form onto the substrate surface by pressure at a temperature around 100.degree. C. The solder mask may then be cured by exposure to UV light. The surface irregularities of the solder mask will be much smaller than those of the substrate therefore providing a comparatively flat surface upon which to place a layer of photosensitive resist. Thus there is provided an inexpensive process in the laminating of the solder mask which is considered both dry and cold and hence is technologically very desirable in manufacturing.